A level of liquid can be detected using a heated negative temperature coefficient (NTC) resistor. For example, U.S. Pat. No. 5,421,202 to Pimpec relates to a sensor that includes a probe including an NTC resistor and a positive temperature coefficient resistor connected in series. A constant current is delivered to the probe and a total resistance of the resistors is compared with a threshold to detect whether liquid is present at a location in a tank. Some liquid level detection systems use a self-heating thermistor probe (e.g. U.S. Pat. No. 4,276,536). U.S. Pat. No. 4,416,153 to Williams is another example of a method that includes comparing voltage levels to thresholds.
A dual thermistor bead sensor can be used to detect a level of liquid. For example, U.S. Pat. No. 6,662,650 to Durkee, et al. relates to conducting different currents through different thermistor beads, measuring voltages across the thermistor beads, generating signals from the voltage measurements, modifying one of the signals by an offset and gain, and detecting a dry/wet state of the sensor based on the modified signal and another generated signal. U.S. Pat. No. 6,758,084 to Hall is another example.
Liquid level detection methods as in the examples are useful in limited temperature ranges. When the temperature range is large, and the environment is potentially explosive, present solutions may not be fully satisfactory.
Other methods and apparatus compensate a thermistor bead for temperature. U.S. Pat. No. 6,822,460 to Pelkey relates to detecting a dry/wet state of a thermistor bead using temperature compensation. A thermistor bead is disposed at a height in a fuel tank. A temperature sensor is disposed in the fuel tank and can measure a temperature in proximity to the thermistor bead. One circuit conducts a constant bias current of less than thirty milliamps into the fuel tank and through the thermistor bead. A second circuit generates a reference voltage in proportion to the measured temperature of the sensor. A third circuit measures a voltage across the thermistor bead in response to the bias current, and can detect the dry/wet state of the thermistor bead based on the measured and reference voltages.
Methods using temperature compensation, however, may require several wires and sophisticated electronics, such as current sources providing polarization current.
In some environments in which a tank is located—such as in an aircraft—weight is a concern, as is the presence of several wires. It is also desirable for tank components to be simple for reliability, compatibility, accessibility purposes. A system using an NTC polarized by current, in contrast, may use a current source that is relatively complex to design.
In connection with aircraft, the Federal Aviation Administration (FAA) recently defined an acceptable maximum current and maximum temperature for in aircraft tank applications. These values are twenty five milliamps and two hundred degrees Celsius, respectively. Liquid sensor systems are desirable that can operate within these requirements and that involve simple-to-design components and fewer wires.